1. Field
This disclosure relates to electric propulsion for satellites and other spacecraft and to field emission electric propulsion (FEEP) thrusters in particular.
2. Description of the Related Art
Satellites and other spacecraft are launched using primary chemical rocket motors and then use secondary thrusters for navigation, orientation, and (in the case of satellites) orbit maintenance. The secondary thrusters require some form of fuel. The mass of the fuel and fuel-containing structures may be more than 50% of the total orbited mass (the mass the initially reaches orbit) of a satellite. Improving the efficiency of the secondary thrusters would allow less fuel mass for a specific orbital mission and thus potentially reduce the cost of constructing and launching a satellite.
The relative efficiency of rocket motors and thrusters is commonly defined by specific impulse (ISP) which is roughly the number of seconds that one pound of fuel will produce one pound of thrust. Solid-fuel and liquid-fuel chemical rocket motors typically have specific impulse values from 150 to 400 seconds. The specific impulse of electric propulsion thrusters may be several thousand seconds or more.
Rocket motors and thrusters produce thrust by expelling mass. In the case of chemical rocket motors, the expelled mass is expanding combustion gases produced by burning a liquid or solid fuel. In the case of an FEEP thruster, the expelled mass is droplets or ions of a metal which are extracted from a reservoir of liquid metal “fuel” and accelerated by a high electric field. More specifically, droplets or ions of the liquid metal are extracted from the tip of a microscopic emitter needle which is coated with the liquid metal by surface tension or other mechanism. FEEP thrusters using a single emitter needle may produce a very low thrust level. Higher thrust levels may be produced by FEEP thrusters that incorporate a large plurality of emitter needles.
Throughout this description, elements appearing in figures are assigned three-digit reference designators, where the most significant digit is the figure number and the two least significant digits are specific to the element. An element that is not described in conjunction with a figure may be presumed to have the same characteristics and function as a previously-described element having a reference designator with the same least significant digits.